Don't look for the binary digest when pulling layers

This code path is usually never triggered because
the annotations are present; and it was broken until recently.

Remove it to simplify the code and analysis.

Signed-off-by: Miloslav Trmač <[email protected]>

pkg/chunked/compression_linux.go

@@ -132,48 +132,16 @@ func readEstargzChunkedManifest(blobStream ImageSourceSeekable, blobSize int64,
 	return manifestUncompressed, tocOffset, nil
 }
 
-// readZstdChunkedManifest reads the zstd:chunked manifest from the seekable stream blobStream.  The blob total size must
-// be specified.
-// This function uses the io.github.containers.zstd-chunked. annotations when specified.
-func readZstdChunkedManifest(blobStream ImageSourceSeekable, blobSize int64, tocDigest digest.Digest, annotations map[string]string) ([]byte, []byte, int64, error) {
-	footerSize := int64(internal.FooterSizeSupported)
-	if blobSize <= footerSize {
-		return nil, nil, 0, errors.New("blob too small")
+// readZstdChunkedManifest reads the zstd:chunked manifest from the seekable stream blobStream.
+func readZstdChunkedManifest(blobStream ImageSourceSeekable, tocDigest digest.Digest, annotations map[string]string) ([]byte, []byte, int64, error) {
+	offsetMetadata := annotations[internal.ManifestInfoKey]
+	if offsetMetadata == "" {
+		return nil, nil, 0, fmt.Errorf("%q annotation missing", internal.ManifestInfoKey)
 	}
 
-	var footerData internal.ZstdChunkedFooterData
-
-	if offsetMetadata := annotations[internal.ManifestInfoKey]; offsetMetadata != "" {
-		var err error
-		footerData, err = internal.ReadFooterDataFromAnnotations(annotations)
-		if err != nil {
-			return nil, nil, 0, err
-		}
-	} else {
-		chunk := ImageSourceChunk{
-			Offset: uint64(blobSize - footerSize),
-			Length: uint64(footerSize),
-		}
-		parts, errs, err := blobStream.GetBlobAt([]ImageSourceChunk{chunk})
-		if err != nil {
-			return nil, nil, 0, err
-		}
-		var reader io.ReadCloser
-		select {
-		case r := <-parts:
-			reader = r
-		case err := <-errs:
-			return nil, nil, 0, err
-		}
-		footer := make([]byte, footerSize)
-		if _, err := io.ReadFull(reader, footer); err != nil {
-			return nil, nil, 0, err
-		}
-
-		footerData, err = internal.ReadFooterDataFromBlob(footer)
-		if err != nil {
-			return nil, nil, 0, err
-		}
+	footerData, err := internal.ReadFooterDataFromAnnotations(annotations)
+	if err != nil {
+		return nil, nil, 0, err
 	}
 
 	if footerData.ManifestType != internal.ManifestTypeCRFS {

pkg/chunked/internal/compression.go

@@ -200,6 +200,11 @@ func ZstdWriterWithLevel(dest io.Writer, level int) (*zstd.Encoder, error) {
 }
 
 // ZstdChunkedFooterData contains all the data stored in the zstd:chunked footer.
+// This footer exists to make the blobs self-describing, our implementation
+// never reads it:
+// Partial pull security hinges on the TOC digest, and that exists as a layer annotation;
+// so we are relying on the layer annotations anyway, and doing so means we can avoid
+// a round-trip to fetch this binary footer.
 type ZstdChunkedFooterData struct {
 	ManifestType uint64
 

pkg/chunked/storage_linux.go

@@ -314,7 +314,7 @@ func makeConvertFromRawDiffer(ctx context.Context, store storage.Store, blobDige
 }
 
 func makeZstdChunkedDiffer(ctx context.Context, store storage.Store, blobSize int64, tocDigest digest.Digest, annotations map[string]string, iss ImageSourceSeekable, storeOpts *types.StoreOptions) (*chunkedDiffer, error) {
-	manifest, tarSplit, tocOffset, err := readZstdChunkedManifest(iss, blobSize, tocDigest, annotations)
+	manifest, tarSplit, tocOffset, err := readZstdChunkedManifest(iss, tocDigest, annotations)
 	if err != nil {
 		return nil, fmt.Errorf("read zstd:chunked manifest: %w", err)
 	}
@@ -1701,7 +1701,7 @@ func (c *chunkedDiffer) ApplyDiff(dest string, options *archive.TarOptions, diff
 		if tocDigest == nil {
 			return graphdriver.DriverWithDifferOutput{}, fmt.Errorf("internal error: just-created zstd:chunked missing TOC digest")
 		}
-		manifest, tarSplit, tocOffset, err := readZstdChunkedManifest(fileSource, c.blobSize, *tocDigest, annotations)
+		manifest, tarSplit, tocOffset, err := readZstdChunkedManifest(fileSource, *tocDigest, annotations)
 		if err != nil {
 			return graphdriver.DriverWithDifferOutput{}, fmt.Errorf("read zstd:chunked manifest: %w", err)
 		}

pkg/chunked/zstdchunked_test.go

@@ -153,7 +153,7 @@ func TestGenerateAndParseManifest(t *testing.T) {
 	tocDigest, err := toc.GetTOCDigest(annotations)
 	require.NoError(t, err)
 	require.NotNil(t, tocDigest)
-	manifest, _, _, err := readZstdChunkedManifest(s, 8192, *tocDigest, annotations)
+	manifest, _, _, err := readZstdChunkedManifest(s, *tocDigest, annotations)
 	if err != nil {
 		t.Error(err)
 	}